Hi, guys! Thanks for all the great replies! My knee problem is due to being too overweight for a period combined with surgery. If I run, especially on a hard surface, I get pains in my knee very quickly. That is why I avoid running. If I do run, it is short bursts, and that works great. I have an elliptical machine here at work, but I never tried it. I will give it a go. As of swimming, I don't have time to go to a pool, but I will swim more this summer I will look into HIIT and other interval sessions! Again, thank you all (Yesterday I had a brilliant stamina session while instructing: I was completely exhausted, and so was all my students )